1. Field of the Invention
The present invention relates to an image forming device and a method for an electrophotographic process, and, in particular, to a digital image forming device which forms an electrostatic latent image on a photoreceptor and a method of forming this electrostatic latent image by beam modulation of digital image data from a computer or the like.
2. Description of the Prior Art
In recent years, in the field of electrophotographic copying in which an electrostatic latent image is formed on a photoreceptor and the latent image is developed to obtain a visible image, a large amount of research and development has been conducted into methods of forming quality images by adopting a digital method with which it is possible to form high quality images in which improved image quality, conversion, and editing are easily accomplished.
In this image forming method, a digital image signal from a computer or a copy document is spot exposed on a photoreceptor, which is uniformly charged by means of a luminous element such as a laser, an LED array, a liquid crystal shutter, or the like, and formed as dots.
Optical write-in systems which modulate light from a digital image signal which have been proposed are (1) a device in which an acousto-optic modulator is interposed in a laser optical system and light is modulated by means of an acousto-optic modulator; and (2) a device using a semiconductor laser in which the laser beam intensity is directly modulated.
A beam which is modulated by a digital image signal has a circular or oval brilliancy distribution which is close to the shape of a normal distribution curve in which the base extends laterally. For example, a semiconductor laser beam usually exhibits an extremely narrow circular or oval pulse width in either the main scanning direction or the secondary scanning direction or both, on a photoreceptor with a 1 to 6 mW brilliance.
However, even when a desirable electrostatic latent image formed by a beam of this type is developed by reversal developing and a dot image formed, time and time again an image of poor clarity is obtained.
In a device in which the beam strength is direct multivalued modulated, the beam intensity change is large versus the drive current of the semiconductor laser, and with heat and the like causing a poor environment, there is a tendency toward fluctuation. In addition, the response for the drive signal is a problem, and it is necessary to make compensation in the circuit. For this reason, pulse width modulation rather than intensity modulation is used exclusively as the multivalued modulation method (see Japanese Laid Open Patent Application 62-39976).
These photoreceptors usually have a high sensitivity in the initial stages of exposure. This causes a tendency toward fluctuation in the photoreceptor so that a clear, dot-shaped latent image is not formed.
There are reasons, according to the type of image, for example, for attaching more importance to resolution rather than halftone reproduction for characters and drawings, and also reasons for attaching more importance to halftone reproduction for scenery and human images. In this field, methods such as the dither method or the density pattern method are used, and reproduction of a halftone image has been considered.
However, there are documents on which halftone images and line images are mixed on the same document. In this type of document it is necessary that image reproduction be carried out in which importance is placed on both gradation reproducing and resolution depending on the type of the image.
However, as photoreceptors used in this type of image forming device, there are a low-.gamma. type light damping photoreceptor which shows so-called low-.gamma. type light damping characteristics in which light damping, as shown in FIG. 1A, is large in the early stages of exposure, and slow in the middle stages of exposure, and a high-.gamma. type light damping photoreceptor which has the feature of high-.gamma. type light damping characteristics which are fast in the middle stages of exposure, as shown in FIG. 1B.
Low-.gamma. type light damping photoreceptors with a double-layer configuration are commonly known which utilize a charge-developing layer of the Se, CdS or the like single layer type, which is usually used with OPC, and a charge transport layer. However, many photoreceptors which show the abovementioned semiconductor characteristics, when in a low electric field rather than a high electric field usually show low light sensitivity, so when the amount of light declines the potential drops, and, at the same time, the light sensitivity drops. For this reason, an exclusive model of photoreceptor is used in an analogue copying machine for gradation reproducing.
However, because a photoreceptor which shows low-.gamma. type light damping characteristics has high sensitivity in the early stages of exposure, and low sensitivity in the middle stages of exposure, the potential distribution of a dot type of image is wide, and because the image is unclear, from the image density distribution of the dots, the dots have a wide base and an indistinct contour.
Therefore, in a device in which direct multivalue modulation of the beam intensity is performed, the beam intensity change is large with respect to the drive current of the semiconductor layer. There is a tendency toward fluctuation caused by an environment such as heat or the like and, in addition, there is a problem relating to the response to a drive signal, so that circuit compensation is required. For this reason, pulse width modulation rather than intensity modulation is exclusively used as a multivalue modulation method.
On the other hand, because light damping is steep in a high-.gamma. type photoreceptor in the middle stages of exposure, in the case where an analogue copying machine is used, gradation reproducing is inadequate. In addition, because there is the drawback that the light damping curve is changed and deteriorates from the repeated action of this process, this method cannot be effectively utilized in an image forming device.